Re: PHOTO-3D digest 1362

[P3D Bob Howard <bobh@xxxxxxxxxxxxxxxxxxxx>]

Re: Exposure tables and lightmeter..
Dt. T mentions "as if one could not screw up with a light meter!"

It is not only easy for the user to forget to set in a film speed, or 
not know how to interpet the reading it it has to be trasfered to a 
scale, but lightmeters are themselves quite capable of built-in errors.
I was delighted when Tim quoted a descendent of Confucious..to pardody 
the idea that one knows the time until one gets two watches, to apply to 
photo meters.
Two meters rarely give the same reading because they usually intergrate 
a different field of view if different make. (The view is supposed to 
integrate to equivalent of 18% gray..but may not!). CdS cells in later 
meters are notorious for extreme red and IR sensitivity and quirky 
linearity. (At the Japan Olympics the photographer with a contract to 
provide photography tried to get 100 Pentax cameras (then the 
Spotmatic). Pentax could not make 100 cameras to meet his specs on 
exposure because of the CdS problem. Minolta had patented a matrix 
camera meter but could never put it into practice as the grid could 
never got more than 3 CdS element to have close linearity. Only the 
silicon cells and later allowed makers to make the matrix meter. The 
old Selenium cell in a Weston or GE is far better, but such low 
sensitity as it generates electricy directly and needs area and a 
micrometer to read even stong light..but had sensitivy that is nearer 
vision or filter pan film. 
I do use meters now and then and of course automatic cameras that have 
them but check with the Sunny16 rule and its offsets. For low light no 
meter handles reciprocity failure which can start with 1 sec. exposure.
A matrix type could in prinicple have factors for this in its lookup 
table. BobH


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